Force gage measurements to characterize bed mobility in two restored reaches of the lower Merced River, California

Force gage measurements were used to parameterize a force balance model for the initation of sediment motion and thus characterize bed mobility for two reaches of the lower Merced River in California's Central Valley. These data were acquired to support research intended to evaluate the extent to which large-scale restoration projects provided improved salmon spawning habitat. A related goal of this study was to improve our understanding of the geomorphic factors influencing spawning site selection by salmon. The force gage measurements were made at the Merced River Ranch and Robinson Reach field sites and involved using a spring-resisting force gage to push a given submerged particle in a bed-parallel orientation and recording the minimum force necessary to initiate sediment motion, following the methods of Johnston et al., [1998]. For each particle, we also recorded the grain size class (22, 32, 45, 64, 90 or 128 mm), the length of the a, b and c axes of the sediment grain, and the particle's weight. These values were used to parametrize the force balance model of Wiberg and Smith [1987].